Method and apparatus for improved antenna isolation for per-antenna training using variable scaling

ABSTRACT

Methods and apparatus are provided for per-antenna training in a multiple antenna communication system having a plurality of transmit antenna branches. A long training sequence is transmitted on each of the transmit antenna branches such that only one of the transmit antenna branches is active at a given time. The active transmit antenna branch transmits the long training sequence with an increased power level relative to a transmission of a payload by the active transmit antenna branch. The increased power level for the active transmit antenna branch compensates for the inactive transmit antenna branches being silent during the given time. Thus, the active transmit antenna branch provides approximately the same antenna power while transmitting the long training sequence as a total power of the plurality of transmit antenna branches during a transmission of the payload. The increased power level can be provided, for example, by a digital-to-analog converter associated with the active transmit antenna branch.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is related to United States Patent Application,entitled “Method and Apparatus for Improved Antenna Isolation forPer-Antenna Training Using Transmit/Receive Switch,” (Attorney DocketNo. Campbell 3-26), filed contemporaneously herewith and incorporated byreference herein.

FIELD OF THE INVENTION

The present invention relates generally to multiple antenna wirelesscommunication systems, and more particularly, to preamble trainingtechniques for a multiple antenna communication system.

BACKGROUND OF THE INVENTION

Multiple transmit and receive antennas have been proposed to provideboth increased robustness and capacity in next generation Wireless LocalArea Network (WLAN) systems. The increased robustness can be achievedthrough techniques that exploit the spatial diversity and additionalgain introduced in a system with multiple antennas. The increasedcapacity can be achieved in multipath fading environments with bandwidthefficient Multiple Input Multiple Output (MIMO) techniques. A multipleantenna communication system increases the data rate in a given channelbandwidth by transmitting separate data streams on multiple transmitantennas. Each receiver receives a combination of these data streams onmultiple receive antennas.

In order to properly receive the different data streams, receivers in amultiple antenna communication system must acquire the channel matrixthrough training. This is generally achieved by using a specifictraining symbol, or preamble, to perform synchronization and channelestimation. It is desirable for multiple antenna communication system toco-exist with legacy single antenna communications systems (typicallyreferred to as Single Input Single Output (SISO) systems). Thus, alegacy (single antenna) communications system must be able to interpretthe preambles that are transmitted by multiple antenna communicationsystems. Most legacy Wireless Local Area Network (WLAN) systems basedupon OFDM modulation comply with either the IEEE 802.11a or IEEE 802.11g standards (hereinafter “IEEE 802.11a/g”). Generally, the preamblesignal seen by the legacy device should allow for accuratesynchronization and channel estimation for the part of the packet thatthe legacy device needs to understand. Previous MIMO preamble formatshave reused the legacy training preamble to reduce the overhead andimprove efficiency. Generally, the proposed MIMO preamble formatsinclude the legacy training preamble and additional long trainingsymbols, such that the extended MIMO preamble format includes at leastone long training symbol for each transmit antenna or spatial stream.

A number of frame formats have been proposed for evolving multipleantenna communication systems, such as MIMO-OFDM systems. Existing frameformats provide inaccurate estimations for the MIMO systems, such asinaccurate power measurement or outdated frequency offset and timingoffset information, or fail to provide full backwards compatibility tothe legacy devices of some vendors. In one proposed MIMO frame formatassociated with the 802.11n standard, each transmit antenna sequentiallytransmits one or more long training symbols (LTS), such that only onetransmit antenna is active at a time. Such a per-antenna training schemerequires sufficient transmit antenna isolation in the PHY architecturefor MIMO channel estimation during the long training sequence. Thus,while the active antenna is transmitting, the remaining transmitantennas must be “silent” for the receiver to properly obtain thechannel coefficients from the received signals. Proper isolation of oneantenna and its transmitter chain to another is critical to avoidexcessive RF leakage onto the “silent” transmitters, resulting incorrupted channel estimation from the desired transmitter.

In one prior isolation technique, the “silent” transmit antenna chains(typically comprising a digital signal processor, RF transceiver andpower amplifier) were switched on and off. Such switching of the antennachains, however, will cause the temperature of the corresponding poweramplifier to increase and decrease, respectively. Generally, suchheating and cooling of the power amplifier will lead to “breathing”effects that cause the transmitted signal to have a phase or magnitudeoffset, relative to the desired signal. In addition, turning off theantenna chain may also cause glitches in the voltage controlledoscillator (VCO) in the RF transceiver as well as excessive delays dueto the start-up time of the power amplifiers.

A need therefore exists for methods and systems for performing channelestimation and training in a MIMO-OFDM system with improved antennaisolation.

SUMMARY OF THE INVENTION

Generally, methods and apparatus are provided for per-antenna trainingin a multiple antenna communication system having a plurality oftransmit antenna branches. According to one aspect of the invention, along training sequence is transmitted on each of the transmit antennabranches such that only one of the transmit antenna branches is activeat a given time. The active transmit antenna branch transmits the longtraining sequence with an increased power level relative to atransmission of a payload by the active transmit antenna branch.

The increased power level for the active transmit antenna branchcompensates for the inactive transmit antenna branches being silentduring the given time. Thus, the active transmit antenna branch providesapproximately the same antenna power while transmitting the longtraining sequence as a total power of the plurality of transmit antennabranches during a transmission of the payload. The increased power levelcan be provided, for example, by a digital-to-analog converterassociated with the active transmit antenna branch.

According to another aspect of the invention, a digital codecorresponding to a binary value of zero is optionally applied to one ormore digital-to-analog converters associated with the inactive transmitantenna branches, for additional isolation. The long training sequencescan be used for MIMO channel estimation. In addition, short trainingsequences can optionally be transmitted substantially simultaneously oneach of the transmit antennas.

A more complete understanding of the present invention, as well asfurther features and advantages of the present invention, will beobtained by reference to the following detailed description anddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of an exemplary MIMO transmitter;

FIG. 2 is a schematic block diagram of an exemplary MIMO receiver;

FIG. 3 illustrates a frame format in accordance with the IEEE 802.11a/gstandards;

FIG. 4 is a schematic block diagram of an exemplary 2×2 MIMO transceiverincorporating features of the present invention; and

FIG. 5 illustrates an exemplary preamble format and power design for anexemplary 4×4 MIMO system incorporating features of the presentinvention.

DETAILED DESCRIPTION

The present invention provides per-antenna techniques for preambletraining for MIMO systems with improved antenna isolation. According toone aspect of the invention, a MIMO “per-antenna-training” preamblealgorithm is disclosed that uses an antenna transmit/receive RF Switchto provide improved isolation of the “silent” antennas from the activeantenna transmitting the long training sequence. According to anotheraspect of the invention, a MIMO “per-antenna-training” preamblealgorithm is disclosed that uses variable scaling of OFDM symbols in thedigital signal processor to give the digital-to-analog converterssufficient dynamic baseband signal power range for transmittingindividual, higher power LTS. Generally, as discussed further below inconjunction with FIGS. 4 and 5, the power level of the activetransmitter is increased during transmission of the LTS to compensatefor the fact that the inactive transmitters are silent during thisinterval.

FIG. 1 is a schematic block diagram of a MIMO transmitter 100. As shownin FIG. 1, the exemplary two antenna transmitter 100 encodes theinformation bits received from the medium access control (MAC) layer andmaps the encoded bits to different frequency tones (subcarriers) atstage 105. For each transmit branch, the signal is then transformed to atime domain wave form by an IFFT (inverse fast Fourier transform) 115. Aguard interval (GI) of 800 nanoseconds (ns) is added in the exemplaryimplementation before every OFDM symbol by stage 120 and a preamble of32 μs is added by stage 125 to complete the packet. The digital signalis then pre-processed at stage 128 and converted to an analog signal byconverter 130 before the RF stage 135 transmits the signal on acorresponding antenna 140.

FIG. 2 is a schematic block diagram of a MIMO receiver 200. As shown inFIG. 2, the exemplary two antenna receiver 200 processes the signalreceived on two receive antennas 255-1 and 255-2 at corresponding RFstages 260-1, 260-2. The analog signals are then converted to digitalsignals by corresponding converters 265. The receiver 200 processes thepreamble to detect the packet, and then extracts the frequency andtiming synchronization information at synchronization stage 270 for bothbranches. The guard interval (GI) is removed at stage 275. The signal isthen transformed back to the frequency domain by an FFT at stage 280.The channel estimates are obtained at stage 285 using the long trainingsymbol. The channel estimates are applied to the demapper/decoder 290,and the information bits are recovered.

FIG. 3 illustrates a frame format 300 in accordance with the IEEE802.11a/g standards. As shown in FIG. 3, the frame format 300 comprisesten short training symbols, t1 to t10, collectively referred to as theShort Preamble. Thereafter, there is a Long Preamble, consisting of aprotective Guard Interval (GI) and two Long Training Symbols, T1 and T2.A SIGNAL field is contained in the first information bearing OFDMsymbol, and the information in the SIGNAL field is needed to transmitgeneral parameters such as packet length and data rate. The ShortPreamble, Long Preamble and Signal field comprise a legacy header 310.The OFDM symbols carrying the DATA follows the SIGNAL field.

The preamble includes two parts, the training part and the signal field.The training part allows the receiver 200 to perform packet detection,power measurements for automatic gain control (AGC), frequencysynchronization, timing synchronization and channel estimation. Thesignal field is going to be transmitted in the lowest rate and givesinformation, for example, on data rate and packet length. In the MIMOsystem, the signal field should also indicate the number of spatialstreams and the number of transmit antennas 140.

The receiver 200 uses the preamble to get all the above information inthe preamble. Based on this information, when the data arrives, thereceiver 200 removes the GI and transforms the data into the frequencydomain using FFT, de-interleaves and decodes the data.

As previously indicated, in a MIMO system, besides these functions, itis also preferred that the preamble be backwards compatible with thelegacy 802.11a/g devices, i.e., the legacy device should be able to getcorrect information about the duration of the packet so that it canbackoff correctly and does not interrupt the MIMO HT transmission.

An exemplary frame format incorporating features of the presentinvention is as discussed further below in conjunction with FIG. 5.

Antenna Isolation for Per-Antenna Training

As previously indicated, the long training sequence (LTS) in thepreamble is used during channel estimation to obtain the m-TX by n-RXchannel coefficients from each individual transmit antenna to each ofn-receive antennas. The other transmit antennas must be silent when theactive transmit antenna is transmitting the long training sequence. Forexample, the inactive antennas can be considered to be “silent” as longas the power of the inactive antennas is reduced by 30 dB, relative tothe active antenna.

According to one aspect of the invention, a MIMO “per-antenna-training”preamble algorithm is disclosed that uses an antenna transmit/receive RFSwitch to provide improved isolation of the “silent” antennas from theactive antenna transmitting the long training sequence. According toanother aspect of the invention, a MIMO “per-antenna-training” preamblealgorithm is disclosed that uses variable scaling of the OFDM symbols inthe digital signal processor to give the digital-to-analog converterssufficient dynamic baseband signal power range for transmittingindividual, higher power LTS. Generally, as discussed further below inconjunction with FIGS. 4 and 5, the power level of the activetransmitter is increased during transmission of the LTS to compensatefor the fact that the inactive transmitters are silent during thisinterval.

In a further variation, a O-code can optionally be applied to thedigital-to-analog converter(s) in the transmit antenna chain for thesilent antennas. In this manner, the RF transceiver and power amplifiercan remain turned on during the silent period. This will avoid VCOglitching problems and extra power amplifier start-up transients.

FIG. 4 is a schematic block diagram of an exemplary 2×2 MIMO transceiver400 incorporating features of the present invention. As shown in FIG. 4,the exemplary transceiver 400 comprises a baseband chip 410, an RFtransceiver 450 and a power amplifier chip 480. The baseband chip 410 iscomprised of a digital signal processor 415 and a number ofdigital-to-analog converters 420-1 through 420-4 (hereinafter,collectively referred to as digital-to-analog converters 420). Thedigital signal processor 415 generates the digital values to betransmitted, in a known manner. The digital-to-analog converters 420convert the digital values into analog values for transmission. Duringtransmission of the long training sequence, the DSP 415 generates thedigital value corresponding to the long training sequence for the activeantenna and generates a 0-code to be applied to the digital-to-analogconverter(s) 420 for the silent transmit antennas. Applying a 0-code tothe digital-to-analog converter(s) 420 for the silent transmit antennassignificantly reduces the RF power out of the power amplifiers 485,discussed below. In the exemplary two antenna embodiment shown in FIG.4, the first antenna 490-1 is active, while the second antenna 490-2 issilent.

In addition, as discussed further below, the DSP 415 generates a controlsignal TXON that controls the position of a transmit/receive switch 490associated with each antenna branch.

The exemplary RF transceiver 450 is comprised of low pass filters 460-1through 460-4, mixers 470-1 and 470-2, and drivers 475-1 through 475-4.The RF transceiver 450 operates in a conventional manner. Generally, inthe exemplary embodiment of FIG. 4, the digital-to-analog converters 420generate in-phase (I) and quadrature (Q) signals that are applied to thelow pass filters 460-1 through 460-4. The filtered signals are appliedto dual band mixers (such as 2.4 GHz and 5 GHz). The drivers associatedwith each antenna branch then operate in an associated band. Forexample, driver 475-1 can operate in a 2.4 GHz band and driver 475-2 canoperate in a 5 GHz band. Likewise, driver 475-3 can operate in a 2.4 GHzband and driver 475-4 can operate in a 5 GHz band.

The power amplifier chip 480 is comprised of a number of poweramplifiers 485-1 through 485-4 that can operate in a conventionalmanner. The output of the power amplifiers 485-1 and 485-2 are appliedto a transmit/receive switch 490-1 associated with the first antennabranch and the output of the power amplifiers 485-3 and 485-4 areapplied to a transmit/receive switch 490-2 associated with the secondantenna branch.

Generally, when in a transmit mode, the transmit/receive switches 490are configured to couple the corresponding power amplifiers 485 to thecorresponding antenna 495. Likewise, when in a receive mode, thetransmit/receive switches 490 are configured to couple the correspondingantenna 495 to the appropriate decode circuitry (not shown), in a knownmanner.

According to one aspect of the invention, the transmit/receive switches490 are used to improve the isolation of the “silent” antenna(s) fromthe active antenna transmitting the long training sequence for MIMO“per-antenna-training.” In particular, the transmit/receive switch 490for the active antenna transmitting the long training sequence isconfigured in a transmit mode, while the transmit/receive switch(es) 490for the silent antenna(s) are configured in a receive mode. In otherwords, the antenna switch 490 for the silent transmitter(s) connects therespective antenna port to the receiver input port. As indicated above,in an exemplary embodiment, the DSP 415 generates a control signal TXONthat controls the position of the transmit/receive switches 490associated with each antenna branch to implement this feature of thepresent invention.

According to another aspect of the invention, variable scaling of theOFDM symbols is employed in the digital signal processor to give thedigital-to-analog converters sufficient dynamic baseband signal powerrange for transmitting an individual, higher power LTS. As indicatedabove, the power level of the active transmitter is increased duringtransmission of the LTS to compensate for the fact that the inactivetransmitters are silent during this interval. The present invention thusrecognizes that in a per-antenna training implementation, the singleactive MIMO transmitter is essentially acting in a Single Input SingleOutput (SISO) mode. Thus, the single active MIMO transmitter mustprovide approximately the same antenna power while transmitting the LTSas the total MIMO power during the OFDM data symbol payload. For theexemplary a 2-TX×2-RX MIMO system of FIG. 4, this requires the activetransmitter to transmit the LTS with 3 dB higher power.

Similarly, for a 4-TX×4-RX MIMO system, this requires the activetransmitter to transmit the LTS with 6 dB higher power. Thus, thedigital-to-analog converters 420 for the active chain have an outputsignal level for transmitting the LTS this is +6 dB higher than employedfor transmitting other fields. When the output power level of thedigital-to-analog converters 420 is +6 dB higher, the antenna power istherefore 6 dB higher as well. The DACs for the “silent” transmitter areat O-code. The variable scaling of the LTS transmission is seen mostclearly in FIG. 5, discussed hereinafter.

In one implementation, the digital signal processor 415 has digitalvariable scaling of the average power of the OFDM symbols to give thebaseband output signal of the digital-to-analog converters 420 avariable range of, for example, 10*log(m) dB, where m equals the numberof MIMO transmit antennas (TX). In this manner, a fast but stable powerramp up/down is provided without the power amplifier gain step transientand bias settling time issues associated with prior techniques. Also, anaccurate antenna power step is obtained without additional power controlrange or complexity needed for the RF transceiver 450.

The LTS has a lower modulated signal Peak-to-Average Power Ratio (PAPR)than the MIMO OFDM data symbols. Thus, the overall PHY transmitterarchitecture of FIG. 4 has sufficient linearity to transmit the LTS withhigher average power in a SISO mode for per-antenna training with lessback-off from the saturated power level. This allows the average outputof the digital-to-analog converters 420 to be 10*log(m) dB larger forthe LTS.

FIG. 5 illustrates an exemplary preamble format and power design for anexemplary 4×4 MIMO system incorporating features of the presentinvention. As shown in FIG. 5, all four transmitters TX1-TX4 aresimultaneously active for transmission of the short training sequence(STS) during an interval 505. Based on the key 550 shown in FIG. 5, eachtransmitter TX1-TX4 is transmitting the STS with a typical power levelof +14 dBm in the exemplary embodiment. At the beginning of the STSinterval 505, the control signal, TXON, for the transmit/receiveswitches 490 is activated to put all transmit branches TX1-TX4 in atransmit mode. In addition, a 2 microsecond delay allows the poweramplifiers to ramp up until the digital-to-analog converters 420 startgenerating the STS symbols.

Thereafter, the LTS is transmitted by each transmitter in a per-antennamanner, as described above. Thus, a first active transmitter TX1transmits the LTS during an interval 510, while the other transmittersTX2-TX4 are silent. The active transmitter TX1 transmits the LTS with anincreased power level of 20 dBm in accordance with the invention.Meanwhile, the silent transmitters TX2-TX4 transmit with a power levelof −10 dBm or less during the silent mode. During interval 510, thecontrol signals, TXON, for each of the transmit/receive switches 490 iscontrolled to put the first transmit branch TX1 in a transmit mode, andthe remaining transmit branches TX2-TX4 in a receive mode.

During a second LTS interval 515, the second transmitter TX2 is activeand transmits the LTS with an increased power level of 20 dBm, while theother transmitters TX1, TX3, TX4 are silent with a power level of −10dBm or less during the silent mode. During a third LTS interval 520, thethird transmitter TX3 is active and transmits the LTS with an increasedpower level of 20 dBm, while the other transmitters TX1, TX2, TX4 aresilent with a power level of −10 dBm or less during the silent mode.During a fourth LTS interval 525, the fourth transmitter TX4 is activeand transmits the LTS with an increased power level of 20 dBm, while theother transmitters TX1, TX2, TX3 are silent with a power level of −10dBm or less during the silent mode.

Finally, after the MIMO preamble is completed, all transmitters TX1-TX4transmit the data symbols during interval 530 with a proper backoff(i.e., using the typical power level of +14 dBm.

While exemplary embodiments of the present invention have been describedwith respect to digital logic blocks, as would be apparent to oneskilled in the art, various functions may be implemented in the digitaldomain as processing steps in a software program, in hardware by circuitelements or state machines, or in combination of both software andhardware. Such software may be employed in, for example, a digitalsignal processor, micro-controller, or general-purpose computer. Suchhardware and software may be embodied within circuits implemented withinan integrated circuit.

Thus, the functions of the present invention can be embodied in the formof methods and apparatuses for practicing those methods. One or moreaspects of the present invention can be embodied in the form of programcode, for example, whether stored in a storage medium, loaded intoand/or executed by a machine, or transmitted over some transmissionmedium, wherein, when the program code is loaded into and executed by amachine, such as a computer, the machine becomes an apparatus forpracticing the invention. When implemented on a general-purposeprocessor, the program code segments combine with the processor toprovide a device that operates analogously to specific logic circuits.

It is to be understood that the embodiments and variations shown anddescribed herein are merely illustrative of the principles of thisinvention and that various modifications may be implemented by thoseskilled in the art without departing from the scope and spirit of theinvention.

1. A method for per-antenna training in a multiple antenna communicationsystem having a plurality of transmit antenna branches, said methodcomprising the step of: transmitting a long training sequence on each ofsaid transmit antenna branches such that only one of said transmitantenna branches is active at a given time, wherein said active transmitantenna branch transmits said long training sequence with an increasedpower level relative to a transmission of a payload by said activetransmit antenna branch.
 2. The method of claim 1, wherein saidincreased power level for said active transmit antenna branchcompensates for said inactive transmit antenna branches being silentduring said given time.
 3. The method of claim 1, wherein said activetransmit antenna branch provides approximately the same antenna powerwhile transmitting said long training sequence as a total power of saidplurality of transmit antenna branches during a transmission of saidpayload.
 4. The method of claim 1, wherein said increased power level isprovided by a digital-to-analog converter associated with said activetransmit antenna branch.
 5. The method of claim 1, further comprisingthe step of applying a digital code corresponding to a binary value ofzero to one or more digital-to-analog converters associated with saidinactive transmit antenna branches.
 6. The method of claim 1, furthercomprising the step of applying a digital code associated with said longtraining sequence to one or more digital-to-analog converters associatedwith said active transmit antenna branch.
 7. The method of claim 1,wherein each of said inactive transmit antenna branches has a powerlevel below a predefined threshold.
 8. A transmitter in a multipleantenna communication system, comprising: a plurality of transmitantenna branches for transmitting a long training sequence such thatonly one of said transmit antenna branches is active at a given time,wherein said active transmit antenna branch transmits said long trainingsequence with an increased power level relative to a transmission of apayload by said active transmit antenna branch.
 9. The transmitter ofclaim 8, wherein said increased power level for said active transmitantenna branch compensates for said inactive transmit antenna branchesbeing silent during said given time.
 10. The transmitter of claim 8,wherein said active transmit antenna branch provides approximately thesame antenna power while transmitting said long training sequence as atotal power of said plurality of transmit antenna branches during atransmission of said payload.
 11. The transmitter of claim 8, whereinsaid increased power level is provided by a digital-to-analog converterassociated with said active transmit antenna branch.
 12. The transmitterof claim 8, further comprising a digital signal processor for applying adigital code corresponding to a binary value of zero to one or moredigital-to-analog converters associated with said inactive transmitantenna branches.
 13. The transmitter of claim 8, further comprising adigital signal processor for applying a digital code associated withsaid long training sequence to one or more digital-to-analog convertersassociated with said active transmit antenna branch.
 14. The transmitterof claim 8, wherein each of said inactive transmit antenna branches hasa power level below a predefined threshold.
 15. A digital signalprocessor for a multiple antenna communication system, comprising: amemory; and at least one processor, coupled to the memory, operative to:generate a long training sequence for transmission on a plurality oftransmit antenna branches such that only one of said transmit antennabranches is active at a given time, wherein said long training sequenceis transmitted by said active transmit antenna branch with an increasedpower level relative to a transmission of a payload by said activetransmit antenna branch.
 16. The digital signal processor of claim 15,wherein said increased power level for said active transmit antennabranch compensates for said inactive transmit antenna branches beingsilent during said given time.
 17. The digital signal processor of claim15, wherein said active transmit antenna branch provides approximatelythe same antenna power while transmitting said long training sequence asa total power of said plurality of transmit antenna branches during atransmission of said payload.
 18. The digital signal processor of claim15, wherein said increased power level is provided by adigital-to-analog converter associated with said active transmit antennabranch.
 19. The digital signal processor of claim 15, wherein saidprocessor is further operative to apply a digital code corresponding toa binary value of zero to one or more digital-to-analog convertersassociated with said inactive transmit antenna branches.
 20. The digitalsignal processor of claim 15, wherein said processor is furtheroperative to apply a digital code associated with said long trainingsequence to one or more digital-to-analog converters associated withsaid active transmit antenna branch.